Various attempts have been made to design such a rechargeable cell suitable for commercial production but difficulties have been encountered.
It is known that manganese dioxide (MnO.sub.2) electrodes in alkaline electrolytes can be recharged if they are only discharged to the point to which the manganese dioxide is converted to the oxide of trivalent manganese (Mn.sub.2 O.sub.3). If the discharge is continued beyond that level, crystal phases are formed which are different from the electrochemically active, rechargeable gamma manganese oxide which is of rutile structure. Thus, if discharge is continued beyond Mn.sub.2 O.sub.3, a complex oxide of manganese is formed corresponding to the formula Mn.sub.3 O.sub.4. This oxide is not rechargeable. In practice this means that the manganese dioxide electrode should not be discharged below the equivalent of the MnO.sub.1.6 level, or, if the voltage is controlled, not below 0.9 V. In other words, the manganese dioxide cathode may only be used to about 40% of its capability if the rechargeable characteristic is to be retained. Unfortunately, it has been difficult to impose this limit with regard to the ordinary domestic user who has no way of knowing when it is reached.
Alkaline cells having manganese dioxide cathodes usually have powdered zinc anodes. It is common practice to bind the zinc powder together into a composite structure and to add zinc oxide to the electrolyte. References to these facts may be found in, for example, the book "Batteries, Vol 1", edited by K. Kordesch, published by Marcel Dekker, 1974. There have also been rechargeable, alkaline Zn/MnO.sub.2 on the market but these cells were not a success due to the previously noted impossibility of the domestic user knowing when recharging was due.
In an attempt to overcome the above difficulty, cells were developed in which the amount of zinc in the anode was limited so that it was only possible to discharge the manganese dioxide cathode to about 40% of its theoretical capacity so that the customer could not destroy the rechargeable characteristics. Cells of this type were given a limited market release by a Japanese and a Finnish manufacturer. However, the manner of achieving the limitation of discharge--which could also be achieved by measuring the discharge voltage of a cell and activating a relay--did not fully satisfy commercial requirements in respect to overdischarge, overcharge and assurance against short circuit by dendrites during charge.
A further difficulty exists in the physical form of the cathode. This is usually manganese dioxide mixed with graphite. This mix swells and expands during discharge. This causes the resistance of the electrode to increase and in the worst case in a mechanical break up of the cathode. In this reference is made also to "Batteries, Vol. 1", pp. 201-219, by K. Kordesch and to several earlier attempts to achieve better cohesion of the electrodes by the addition of binders such as cement like in U.S. Pat. No. 2,962,540, or by adding graphitized textile fibres as described in U.S. Pat. No. 2,977,401 or by the addition of latex as in U.S. Pat. No. 3,113,050. A far more reaching measure for preventing the decomposition of the electrode during electrical cycling and prevent its tendency to swell takes the form of using a supplemental binder in accordance with U.S. Pat. No. 3,945,847, which uses a supplemental binder which is mixed with colloidal graphite. It essentially ensures the solidity and conductivity of the electrode. It can consist of polymers or copolymers of such materials as e.g., styrenes, butadienes, acrylonitriles, formaldehydes, vinyl alcohols or epoxies which are to be wetted by the electrolyte. A good example is also polysulfone dissolved in chlorinated hydrocarbons and mixed with colloidal graphites. While these binders as described improve the coherence of the cathode the ultimate expansion of the cathode could not always be prevented because the binding strength of the materials were exceeded. These expansions and contractions of the MnO.sub.2 cathode during discharge and charge were measured and investigated in detail in scientific publications by Kordesch, Gsellmann and Tomantschger. As an example we cite the Publication in Electrochemica Acta, Vol. 26, No. 10, pp, 1495-1504, 1981. Accordingly, it is an objective of the invention in respect to the MnO.sub.2 electrode to create conditions in which the discharge and charge characteristics take advantages of all possibilities to restrain the cathodes. U.S. Pat. No. 4,384,029 by K. Kordesch and J. Gsellmann describes the enclosure of the MnO.sub.2 electrode by various means like mechanical cases, tubes, springs or mechanical wedges. While these means prove to be successful in cycling the MnO.sub.2 cathodes with the provision of the zinc limitation in the range of 100 to 200 cycles the restriction of the mechanical cage proved to be a disadvantage in respect to the attainment of high current densities. The electrode interface is restricted by this mechanical means, therefore the current density achievable is limited. It was also found subsequently that rather heavy mechanical structures like tubes have to be used to restrain the expanding MnO2. The use of thicker walled tubes had the definite disadvantage of creating poorly conductive or even empty or gas filled spaces in the holes of the tubes which were noticeable by an increased impendance of the cell after extended cycling. The use of cement e.g., necessitates a relatively high percentage of the cement in the mixture (up to 10%) and therefore a reduction of the amount of the active ingredient. If not enough cement is mixed into the mixture then the structure has a tendency to crumble and crack and the purpose of achieving a coherent electrode is not reached.
It has now been found that if a binder is used for a manganese dioxide/graphite cathode mix and the cathode is retained in a restrictive thin screen structure the various difficulties with respect to physical cathode structure are inhibited. If the restrictive structure is also conductive it acts as an overvoltage reducing catalyst on the surface of the cathode in such a way that, on overcharge, oxygen is produced and can be released to travel to the zinc anode. Thus, an endless process is created keeping the cell from exceeding a certain pressure.